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Journal Article

Indeterminate Outcome of Certain Experiments on Drosophila Populations

Theodosius Dobzhansky and Olga Pavlovsky
Evolution
Vol. 7, No. 3 (Sep., 1953), pp. 198-210
DOI: 10.2307/2405731
Stable URL: http://www.jstor.org/stable/2405731
Page Count: 13
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Indeterminate Outcome of Certain Experiments on Drosophila Populations
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Abstract

Experimental populations of Drosophila pseudoobscura were made which contained mixtures of flies with ST and with CH gene arrangements in their third chromosomes. In some populations all chromosomes were of geographically uniform origin, deriving from wild ancestors collected in a certain locality in California. In other populations the flies of the foundation stock were race hybrids, the carriers of the ST chromosomes descending from California, and those of the CH chromosomes derived from ancestors collected in Mexico. The outcomes of the replicate experiments on population of geographically uniform origin were alike, within the limits of sampling errors. The heterozygotes ST/CH exhibited heterosis, and the populations eventually reached equilibria, with ST and CH chromosomes continuing to occur with predictable frequencies. Replicate populations of geographically mixed origin exhibited a variety of behaviors. The ST/CH heterozygotes in such populations originally possessed no heterosis. But in two out of the six populations heterosis has eventually developed, and the populations reached equilibrium. In four remaining populations heterosis did not develop, and CH chromosomes tended to be eliminated completely. The adaptive values of the ST/ST, ST/CH, and CH/CH genotypes changed with time. In fact, no two of the populations of mixed origin were quite alike. The changes induced by natural selection in populations of geographically mixed origin are more profound than those which occurred in populations of geographically uniform origin. In the former, new adaptively coherent genotypes have been molded by natural selection from the segregation products of the interracial cross. In the latter, only changes in frequencies of preexisting genetic variants have taken place. The former can be referred to as mesoevolutionary and the latter as microevolutionary changes. The predictability and repeatability of mesoevolutionary changes in finite populations are limited.

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